This invention relates to a field effect transistor and a method for manufacturing the same.
Field effect transistors (FET) including thin film transistors (TFT) that have now been used in a variety of electronic appliances are constituted, for example, of a channel-forming region and source/drain regions formed on a silicon semiconductor substrate or silicone semiconductor layer, a gate insulating layer formed on the surface of the silicon semiconductor substrate or silicone semiconductor layer and made of SiO2, and a gate electrode provided in face-to-face relation with the channel-forming region through the gate insulating film. Alternatively, the transistor may be constituted of a gate electrode formed on a substrate, a gate insulating layer formed on the substrate including the gate electrode, and a channel-forming region and source/drain regions formed on the gate insulating layer. For the manufacture of the field effect transistors having such structures, a very expensive semiconductor apparatus has been in use, and thus the reduction of manufacturing costs has been strongly demanded.
In order to meet the demand, attention has been recently made to studies and developments of FET using organic semiconductor materials, with which FET can be manufactured based on methods such as a spin coating method, a printing method, a spraying method, without resorting to vacuum techniques.
FET is required to exhibit high-speed operations because of the requirement of being assembled in many electronic appliances including displays. For instance, FET necessary for this purpose is such that video signals are converted to data as occasion arises and switching operations of on/off can be performed at high speed.
Where organic semiconductor materials are used, the mobility that is a characteristic index of TFT, for example, is obtained as a typical value which is as low as 10−3 to 1 cm2/Vs (see, for example, C. D. Dimitrapopoulos, et at., Adv. Mater. (2002), 14. 99). This value is lower than several cm2/Vs of the mobility of amorphous silicon or about 100 cm2/Vs of the mobility of polysilicon, and thus does not arrives at such a mobility of 1 to 3 cm2/Vs as required for TFT for displays. Accordingly, FET using organic semiconductor materials has a great problem on how to improve mobility.
The mobility of FET using an organic semiconductor material is determined depending on the intramolecular charge transfer and the intermolecular charge transfer. The intramolecular charge transfer becomes possible when atomic orbitals are superposed between adjacent multiple bonds sandwiching a single bond so that electrons are non-localized to form a conjugated system. The intermolecular charge transfer is realized through conduction resulting from the superposition of molecular orbitals caused by intermolecular bond or van der Waals' force or the hopping conduction through an intermolecular trapping level.
In this case, when the intramolecular mobility is taken as μintra, the mobility of intermolecular bond taken as μinter, and the mobility based on the intermolecular hopping conduction taken as μhop, the following relationship is established.μintra>>μinter>μhop
With organic semiconductor materials, the mobility is limited, as a whole, with the slow intermolecular charge transfer, so that the mobility of charge is small.
In order to improve the mobility of FET using organic semiconductor materials, studies have been extensively made.
For instance, where a pentacene thin film which is a kind of organic semiconductor material is formed according to a vacuum deposition technique, the deposition rate during the deposition is suppressed to an extreme extent and the substrate temperature is set at room temperature thereby improving orientation of molecules and arriving at a mobility of 0.6 cm2/Vs (see C. D. Dimitrakopoulos et at., IBM J. Res & Dev. (2001), 45, 11). This method aims at improving the mobility by improving the crystallinity of material and suppressing the intramolecular hopping conduction. Although the mobility is improved, the intramolecular movement limits the mobility as a whole like other types of organic semiconductor materials. Eventually, such a great mobility as to be satisfied cannot be achieved.
For an organic semiconductor transistor positively using intramolecular charge transfer, a self-assembled monolayer field-effect transistor (SAMFET) of Luscent Technology Inc., has been proposed. In this device, a semiconductor layer made of a monolayer is formed between a source electrode and a drain electrode through self-assembling thereby realizing SAMFET having a gate length of 15 nm. In this SAMFET, the channel-forming region is constituted of the monolayer that is oriented along the direction of connecting the source electrode and the drain electrode, so that the charge transfer within the channel-forming region is limited only to the intramolecular movement. As a consequence, a mobility of 290 cm2/Vs that is higher than that of polysilicon has been achieved (see J. H. Schoen et al., Nature (2001), 413, 713; Appl. Phys. Lett (2002), 80, 847). However, such a channel structure has a gate length which is determined depending on the thickness of the monolayer film, so that the gate length becomes so short as several nanometers. This brings about the problem that the pressure resistant between the source and drain regions becomes low, not making it possible to achieve a high drive voltage. For the formation of electrodes on the monolayer film without breakage of the monolayer film, the substrate temperature should be cooled down to −172° C. to −30° C., and thus process costs become high. Thus, this process has little practical merits.
A channel material using a blend of organic and inorganic materials has been proposed in Japanese Patent Laid-open No. 2000-260999. More particularly, while, in the technique disclosed in Japanese Patent Laid-open No. 2000-260999, a layer structure is formed of an inorganic component and an organic component to utilize the high carrier mobility characteristic of the inorganic crystalline solid on one hand and the ability of the organic component assisting in self-assembling of the inorganic material on the other hand, deposition of the material on a substrate under low temperature processing conditions is enabled. Although a mobility of 1 to 100 cm2/Vs has been expected, the mobility actually achieved is as low as 0.25 cm2/Vs. This value is higher than that of an organic semiconductor material generally formed according to a spin coating technique, and is at the same level as that of an organic semiconductor material formed by vacuum deposition or the like. The mobility higher than that achieved by amorphous silicon has never been obtained.